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Old 01-10-2009, 22:43   #1
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AIS, VHF, WIFI, SSB Antenna Location

Where should I put all the differnt Antennas for maximum performance and make the right tradeoffs?

I could use the spreaders on the mast and the two poles on the back. I plan to have a wind generator and a Radar on the two poles.

Given the issues with AIS and VHF stepping on one another when transmitting. Where would you pull all the antennas. I also plan to have a sat phone but since the boat is epoxy composite I'm told I can put the sat antenna in the bow or inside the cabin along with the GPS antenna.

The SSB will go on the back, probably the wind generator pole. Put the AIS on the other pole with the Radar? Put the VHS on the spreader for maximum range and also run the IslandTime Marine WiFi System antenna on a spreader? It's a carbon mast so not sure I really want to be putting holes in it.

Other antenna ideas that would work better? Anyone combining AIS and VHF to their satisfaction?

Here's a picture of a sister ship.

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Old 01-10-2009, 23:55   #2
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VHF: masthead
AIS: on spreader; btw: consider a transponder
SSB: lower support on hull and upper support on post; the whip must be perfectly vertical, not macho angled aft. The feed wire from tuner to whip very short, like 3'
Wifi: max. 20' coax between amplifier and antenna (don't know that islandtime system but it sounds awfully slow). I would recommend an omnidirectional (like Hyperlink marine omni) on side support on the post under the radar.
GPS: I would install the primary GPS antenna outside, on a side support under the windgen. The secondary can go inside.
satphone: outside, windgen post. Stuff like solar panels and metal parts like fittings and boom etc. interfere so satellite antenna's should be able to just "look" over the superstructure to the horizon.

A hole in a carbon mast doesn't weaken it more than a hole in an aluminium mast. The key is to close an unused hole with epoxy. When a hole is filled with a fastener, it doesn't really weaken the mast. You have to take precautions for corrosion though, just like with aluminium masts. Dip the screws/rivets in TefGel before mounting them.

nice boat!

cheers,
Nick.
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Old 02-10-2009, 00:12   #3
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Put the open array radar on the mast head and have the wind indicator in the middle and the VHF on one side and the AIS on the other side. Then tie some streamers to each and a kite off the middle and a couple of balloons.

Not only is it extremely effective but it looks real pretty! Believe me!
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Old 02-10-2009, 08:34   #4
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Quote:
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....
SSB: lower support on hull and upper support on post; the whip must be perfectly vertical, not macho angled aft....
Sorry but I gotta ask, why perfectly vertical?
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Old 02-10-2009, 08:54   #5
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Sorry but I gotta ask, why perfectly vertical?
Because when you "slope" it, it becomes directional. On a boat, the primary antenna should be as omnidirectional as possible and this is where the long whip can gain an advantage over an insulated backstay.
This whip is called a "vertical antenna" which logically implies that it should be mounted vertical.

We don't have backstays so we use a whip on the SSB too. For my HAM radio, I hoist a wire with a halyard up the mizzen, which becomes very much like an insulated backstay. I was pleasantly surprised that the whip performed just as good as the wire.

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Old 02-10-2009, 09:10   #6
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As not to pirate BRC's thread, I will start a new thread to continue the "SSB vertical whip or not" divergence of opinion
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Old 02-10-2009, 19:07   #7
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Originally Posted by MarkJ View Post
Put the open array radar on the mast head and have the wind indicator in the middle and the VHF on one side and the AIS on the other side. Then tie some streamers to each and a kite off the middle and a couple of balloons.

Not only is it extremely effective but it looks real pretty! Believe me!
Thanks I needed a good laugh today!
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Old 02-10-2009, 19:56   #8
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VHF - mast top, can be used with a splitter to serve as the AIS one, too. Think range.

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Old 03-10-2009, 17:09   #9
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VHF - mast top, can be used with a splitter to serve as the AIS one, too. Think range.

b.
I've heard twice and read in Practical Sailor that the loss from splitting the cable will result in less range. Also, won't an antenna with a splitter have the same or worse problems when the AIS and VHF both try to transmit?
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Old 03-10-2009, 17:35   #10
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Not if you use the right specs of cable. However, you might be right for very long cable runs. Mine is 15 meters only. Off course, on a very tall rig (say in excess of 20 meters) it may seem tempting to place it elsewhere - the problem is there are not many better places than the mast top (spreaders will blanket the antenna in some sectors). So my second choice would be the arch - but most arches are just too low.

Then again, how would this loss count if you are at 25 watt output? I believe we 'lose' more range by choosing a short stub (which is best unless on a cat - which IS the case) than by using too long (how long is too long according to the Practical Sailor?) cable run.

I have never heard of any problems with splitters and given the specs of AIS transmissions it could only be a very potential issue with TX and if you are a real chatter-box on air (which is not encouraged by the marine authorities anyway).

In summary:
- if not the mast top - then where???
- AIS and VHF separate antennas - best, but a single one plus splitter a viable option.

A tip for cat sailors - you can use longer antennas and so get longer range than monohulls that will (or not) use short studs! Good on yer mates!

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Old 03-10-2009, 20:47   #11
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For VHF, it's always better to mount the antenna higher. Just use a good coax, RG-8U minimum or better RG-213.

When you have an AIS transponder, you must use a dedicated antenna. When you only have an AIS receiver, it's still better to have a separate antenna. It can double as emergency VHF antenna too.

We have a dipole (as opposed to gound-plane whip) on top the mast head and it's only 4' tall.

cheers,
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Old 05-10-2009, 10:25   #12
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sorry nick, Comar, a very experienced builder of AIS system doesnt agree with you]

see Comar Systems AST 100 Technically with a proper intelligent splitter, theres no need for dual antennas, in fact given that your VHF spends 99.9% of its time NOT transmitting , it makes sense to make better use of that high mounted VHF antena
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Old 05-10-2009, 11:57   #13
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Class 'B' AIS transponders transmit once every three minutes and the signal pulse is considerably less than a second. When you have an intelligent splitter that meets standards the signal loss is is really very low. Any loss is more than compensated for by achieving height. Pay the extra for top quality co-ax to your antenna if you have a long run and you'll never know the difference. IMHO.

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Old 05-10-2009, 13:18   #14
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sorry nick, Comar, a very experienced builder of AIS system doesnt agree with you]

see Comar Systems AST 100 Technically with a proper intelligent splitter, theres no need for dual antennas, in fact given that your VHF spends 99.9% of its time NOT transmitting , it makes sense to make better use of that high mounted VHF antena
Thanks goboatingnos. Seems like the best solution for us. Thanks for finding this splitter. Now I guess the only question is weather or not it's necessary but given we have a 75 foot plus mast it seems like a good idea.
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Old 05-10-2009, 15:10   #15
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I think you're all a bit silly. That Comar switch is a fine product by itself, but it's also $350.- which is much more than the cost of 2nd VHF antenna on the radar post/arch/whatever and that 2nd antenna is still the better solution. I think of it like this:
  • It doesn't matter how little insertion loss it has; it's still insertion loss and you have to add the insertion loss of the extra plugs to that also.
  • It doesn't matter how short a data transmission from your AIS is: when you use your VHF at that moment, your AIS can't transmit (VHF Tx has priority). With a 2nd antenna, the AIS will transmit while you use your VHF simultaneously.
  • When things get tight, you will almost certainly use your VHF to communicate with the other party and that is the moment that it is most important to get your AIS signal out. When your switch blocks that it does so at the worst possible moment. Bad bad bad and this is why it isn't allowed at all for commercial (class A) installations.
  • Your AIS antenna doesn't need to be high up the mast; you don't need ships 10+ nm away to see you.
  • You might consider that expensive gadgets are better but in this case it isn't. Even a cheap 2nd antenna is better, plus it provides a backup antenna for your VHF.
Comar doesn't disagree with me at all, they just put a product on the market that people ask for. When you ask Comar what's better, they will agree that the 2nd antenna is better.

I must admit that this Comar switch is the first one that allows two transmitters while both stations always receive when no transmission is in progress (is that true?). Some readers will probably have made up their mind to buy cheaper switches which don't allow that at all and fry their AIS etc.

cheers,
Nick.
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